FIG. 1 shows a representative, non-limiting, shape for a 3D glass cover (also known in the art as a “3D cover glass”), which can be used with an electronic device, such as, a telephone, television, tablet, monitor, or the like. As shown in this figure, 3D glass cover 100 includes: (i) a planar central portion 101, (ii) a perimeter portion 102, and (iii) a perimeter edge 103.
Planar central portion 101 is flat or nearly flat, i.e., its radius of curvature is at least 300 millimeters. Perimeter portion 102 extends out of the plane of the planar central portion 101, thus providing the glass cover with an overall three-dimensional shape, as opposed to a two-dimensional shape. Although as shown in FIG. 1, perimeter portion 102 completely surrounds central portion 101, in some embodiments, the perimeter portion can extend around only a portion of the central portion, e.g., for a glass cover having a rectangular shape, less than all four sides of the glass cover can include a perimeter portion, e.g., two sides can have a perimeter portion and the other two sides can be flat or substantially flat. Likewise, to be three-dimensional, a glass cover in the form of a disc or saucer only needs to have a portion of its flat or nearly flat central portion transition into a perimeter portion which extends out of the plane of the flat or nearly flat central portion.
As will be evident, the shape of a 3D glass cover can vary widely depending on the desires of the designer of the device with which the 3D glass cover will be used. Thus, the 3D glass cover can have a variety of overall shapes and can include central portions and perimeter portions of various sizes and shapes, and can employ transitions of various configurations between the central and perimeter portions. Commonly-assigned U.S. application Ser. No. 13/774,238 entitled “Cover Glass Article” filed Feb. 22, 2013, published as U.S. Patent Application Publication No. 2013/0323444, the contents of which are incorporated herein by reference, provides various representative dimensions for 3D glass covers, as well as descriptions of typical applications for the covers. The molding technology disclosed herein can be used with 3D glass covers of these types, as well as other types now known or subsequently developed.
The transverse dimension (thickness) of perimeter edge 103 corresponds to the thickness of the glass from which the glass cover is made, which is typically less than 1 millimeter, e.g., 0.8 millimeters or less. Because of this small thickness, prior to the present disclosure, it had been believed that stress changes at the edge could be ignored in predicting changes in the overall shape of a 3D glass cover as a result of ion exchange (IOX) strengthening. In particular, on an area basis, the perimeter edge of a typical glass cover amounts to less than about two percent of the overall area of the part. Hence, the number of ions exchanged through the edge is only a small fraction of the total number of ions exchanged, thus making it reasonable to assume that relative to the total number of ions exchanged, those few ions would have little effect on the structural behavior of the part.
In fact, in accordance with the present disclosure, it has been surprisingly found that ion exchange at the edge not only has a substantial effect on the overall shape, but its effect is in many cases greater than the effect of ion exchange on the rest of the part. Thus, although only a small number of ions move through the edge relative to the total number of ions that move through the surfaces of the rest of the part, those edge-traversing ions are critical to the shape changes exhibited by 3D glass covers as a result of IOX strengthening. In terms of commercial value, this discovery permits manufacturers of 3D glass covers to effectively and efficiently meet tolerance requirements of customers for those covers. In particular, as detailed below, it allows manufacturers of 3D glass covers to produce molds for making those covers which accurately compensate for the changes in shape which the cover will exhibit when IOX strengthened. The technology thus represents a valuable contribution to the ability of designers to create aesthetically pleasing designs for 3D glass covers and the manufacturers of the covers to accurately produce the shapes envisioned by the designers.